1. Field of the Invention
This invention relates to fluoroscopic imaging and more particularly to apparatus for limiting the radiation dosage to an examination object within a limited photon energy range.
2. Description of the Prior Art
It is well known in the art of fluoroscopy, in both medical and industrial applications, to limit or reduce the level of radiation exposure to the examination object by the use of filters. As disclosed in U.S. Pat. No. 3,860,817, during fluoroscopy, the X-ray beam, emitting from a source, is filtered so that substantially only radiation, within a preselected spectral band or window, impinges on the patient or the examination object. Thus, a minimumm of the radiation energy is wasted in a film changer intensifying screen so that the ratio of information output to patient dose is optimized. Preferably, the material for the filter is chosen on the basis of a preselected photon energy band or window, which will pass through the filter and be absorbed by the examination object.
U.S. Pat. No. 2,225,940 discloses a movable wedge-shaped filter made of aluminum. The filter is positioned transversely to the radiation beam, and movement of the filter transversely to the beam presents different thicknesses of the filter to be traversed by the beam. One undesirable feature of the graduated or wedge filters is the hardening of the beam which occurs and the resulting image has a quality degraded to a degree that is unacceptable.
The use of the known graduated filters for filtering the upper critical absorption edge (K-edge energy) spectral range of the X-ray photon energy requires an increase in the X-ray tube current to increase the intensity of the X-ray photon energy emitted from the tube. This has the undesirable consequence of exposing the patient or the examination object to undesirably high radiation dosage during fluoroscopy. Also, increasing the X-ray tube current results in excessive X-ray tube loading. Other examples of fluoroscopy filtration are disclosed in U.S. Pat. Nos. 1,624,443; 2,901,631; 3,402,292; 3,976,889; 4,101,766 and 4,246,488.
Overall, the known fluoroscopy filtration devices have been directed to isolating spectral distributions that exploit absorption characteristics of tissue and contrast media for improved images. However, these efforts have not been effective to optimize the filter material for routine fluoroscopy and, in particular, in the fluoroscopy of soft tissue and bones. Therefore, there is need in fluoroscopy filtration to provide a filter operable in the fluoroscopy of both soft tissue and bones and composed of a material having a critical absorption edge (K-edge energy) that eliminates the photon energy levels of the radiation beam that play little or no role in the imaging process.